1. Field of the Invention
The present invention relates to a polyester composition and a process for its preparation, and more specifically it relates to a polyester composition containing uniformly dispersed particles with an average size of 0.03-1.8 xcexcm, which is useful for molding of fibers, films and the like, and to a process for its preparation. The present invention also relates to an apparatus for preparation of the composition.
2. Description of the Related Art
Polyesters, particularly polyethylene terephthalate, polybutylene terephthalate and polyethylene-2,6-naphthalate, are widely used for fibers, films and other molded products because of their excellent physical and chemical properties. Despite these excellent properties, however, they are known to have problems of molding workability during the molding processes used to obtain such molded products, and to cause troubles including reduced workability due to undesirable slidability during handling of the molded products themselves and lower product quality.
In order to overcome these problems, a number of processes have been proposed whereby fine particles are added to the polyester to provide a suitable degree of irregularity to the molded product surface, to thus reduce the surface slidability of the molded products, and some of these processes have been practically applied. Examples include a process whereby inactive inorganic particles such as silicon oxide, titanium dioxide, calcium carbonate, talc, kaolinite, etc. are added to the polyester (for example, Japanese Unexamined Patent Publication No. 55-133431) and a process whereby heat-resistant polymer particles such as silicon particles or polystyrene particles are added to the polyester (for example, Japanese Unexamined Patent Publication No. 3-115354).
However, most of these processes for adding particles involve causing coarse particles to be contained in the composition. Yet, in the case of magnetic tape films, for example, mixture of coarse particles lowers the electromagnetic conversion characteristics, becoming a cause of dropouts. Also, in the case of films which must be transparent, such as films for plate-printing and microfilms, the transparency is notably reduced thus impairing the quality of the film. Such particles are not preferred for use in production of fibers since they can cause filter clogging during spinning, thus lowering the productivity and resulting in breaking of the monofilaments.
Measures for eliminating such coarse particles have been proposed for cases where particles are added during the synthesis reaction of the polyester, wherein the coarse particles are first removed by crushing and sorting and then added in the form of a slurry or solution. (For example, Japanese Examined Patent Publication No. 1-41170, Japanese Unexamined Patent Publication No. 63-105059). On the other hand, measures have also been proposed for cases where they are kneaded and mixed with the polyester after synthesis, wherein a single-rotary-member or twin-rotary-member extruder is used to add powdered particles while shear stress is applied to the polymer and powder particles. (For example, Japanese Unexamined Patent Publication No. 2-34307, Japanese Examined Patent Publication No. 7-62076).
Nevertheless, the former case not only involves high cost and longer working times for the crushing and sorting procedures, but even after those procedures have been sufficiently carried out, the particles reaggregate during and after the time that they are added to the polyester synthesis system, for which reason it is still difficult to avoid production of coarse particles in the polyester product. In addition, when particles are repeatedly added to a polyester synthesis system designed for mass production of a polyester, tremendous loss is experienced by washing, etc. of the polyester synthesis system each time the polyester brand is switched to a different brand.
On the other hand, in the latter case it is very difficult in practice to completely crush or shred the coarse grains of the powder grains using a single-rotary-member or twin-rotary-member extruder, and when too much shear stress is applied in order to evenly disperse the grains in the polyester, the intrinsic viscosity of the polyester is notably lowered, resulting in the problem of poor moldability or product quality.
In order to solve these difficulties, there have been proposed a process whereby a vented extruder is used to add to the polyester a slurry of inorganic particles of average size 0.01-5 xcexcm in water and/or an organic compound with a boiling point of 200xc2x0 C. or below (Japanese Unexamined Patent Publication No. 3-115352) and a process whereby a co-rotating twin-screw kneading extruder is used for molten kneading of the polyester with titanium oxide of average particle size 0.1-0.5 xcexcm with the particles larger than 3 xcexcm substantially eliminated by wet or dry sorting and/or wet crushing treatment (Japanese Unexamined Patent Publication No. 1-173031); however, a sufficient effect has still not been achieved.
It is an object of the present invention to provide a polyester composition with excellent dispersion of particles in the polyester, containing substantially no coarse particles and thus having excellent molding workability for the making of fiber, film and resin molded products, and which results in low monofilament breakage in the case of fibers, gives excellent slidability, film surface uniformity and abrasion resistance in the case of films, and gives excellent dimensional stability and impact resistance in the case of resin molded products, as well as a process and apparatus for its preparation.
According to the present invention there is provided a polyester composition which consists of polyester (A) defined below containing fine particles with an average size of 0.03-1.8 xcexcm which have been surface-modified with polyester (B) defined below.
Polyester (A): a polyester of which at least 70 mole percent of the repeating units consist of ethylene terephthalate, butylene terephthalate or ethylene naphthalate units.
Polyester (B): a polyester of which less than 70 mole percent of the repeating units consist of ethylene terephthalate, butylene terephthalate or ethylene naphthalate units.
According to the present invention there is also provided a process for preparing a polyester composition which comprises feeding the above-mentioned polyester (A) and fine particles to a single-rotary-member or twin-rotary-member vented extruder and subjecting them to molten kneading, whereby a slurry of the fine particles of average size 0.03-1.8 xcexcm and the above-mentioned polyester (B) is prepared in water and/or an organic compound with a boiling point of 240xc2x0 C. or below, or a slurry of the fine particles with an average size of 0.03-1.8 xcexcm which have been surface-modified with the above-mentioned polyester (B) is prepared in water and/or an organic compound with a boiling point of 240xc2x0 C. or below, and then fed to the above-mentioned vented extruder.
According to the invention there is also provided an apparatus for preparation of the polyester composition, which consists of a vented extruder comprising a polyester resin loading port, a screw for molten kneading of the loaded polyester resin while transporting it, a cylinder which houses the screw and heats the transported polyester resin, and an introducing nozzle for introduction of the fine particle slurry into the heated polyester resin, which apparatus is provided with an outlet for discharge of the medium vapor generated from the slurry introduced into the cylinder, and provided with the introducing nozzle for the slurry in the outlet.